Prior art inkjet printers have regulated ink pressure using feedback control to achieve uniform ink delivery. As shown in FIG. 1, an ink delivery system of this type 10 can use a motor 12 to drive a lead screw 14 that has teeth coupled to a piston 16 that is mounted to slide within a cylinder 18. One or more seals 20 located between the piston and cylinder help to form a tightly sealed, variable-volume chamber 22. The cylinder is also equipped with a pressure transducer 24 that measures the pressure within the chamber, with a supply orifice 26 that receives ink from a reservoir 28 via a check valve 30, and with a delivery orifice that delivers the ink to pen 32 equipped with a nozzle 34. A controller 38 has an input connected to the pressure transducer and an output connected to an input of the motor 12.
The prior art ink delivery system 10 shown begins its operation with the motor 12 causing the lead screw 14 to rotate in its reverse direction. This pulls the piston 16 back out of the cylinder 18 and thereby draws ink from the reservoir 28 through the check valve 30 into the chamber 22. Once the chamber is full, the motor rotates the lead screw in its forward direction. This causes an increase in pressure that shuts the check valve and forces ink to flow out of the nozzle 34. The ink stream is then broken into droplets, which can be deposited onto a print substrate 36 according to well known inkjet printing techniques.
During ink deposition, a feedback control loop keeps pressure in the chamber 22 uniform by causing the controller 34 to modulate its output signal as a function of the pressure signal it receives from the pressure transducer 32. This type of control has been capable of delivering uniform streams of ink for a particular nozzle. But nozzles are often changed in the course of printing operations, and it has been found that normal tolerance variations in nozzle diameter can cause significant differences in drop size, which in turn result in visibly different print output. To address this problem, a dual-mode regulation method was developed.
The dual-mode regulation method uses flow regulation to calibrate the system. When it has settled into a steady state, the controller records the pressure. This recorded pressure is then used as a target pressure in subsequent printing cycles. Steady state is achieved once the parts in the system have had time to settle into their expanded dimensions in the presence of the increased operating pressure and temperature. During the subsequent flow regulation phase, the controller causes the motor to move at a fixed speed, such as by issuing stepper motor step signals at a fixed rate.